As concerns of global climate changes spark initiatives to reduce carbon dioxide emissions, its economic removal from gas streams is becoming increasingly important. Removal by absorption/stripping is a commercially promising technology, as it is well suited to sequester carbon dioxide (CO2). Such carbon dioxide emissions may be produced by a variety of different processes, such as the gas stream produced by coal-fired power plants. The removal of CO2 can be an expensive process, potentially increasing the cost of electricity by 50% or more. Therefore, technology improvements to reduce the costs associated with the removal of CO2 are highly desirable.
The use of absorption and stripping processes with aqueous solvents such as alkanolamines and promoted potassium carbonate is a known, effective technology for the removal and capture of CO2 from flue gas, natural gas, hydrogen, synthesis gas, and other gases. U.S. Pat. Nos. 4,477,419 and 4,152,217, each of which is incorporated herein by reference, describe aspects of this technology. The first generation of technology relating to alkanolamine absorption/stripping uses aqueous solutions of monoethanolamine (MEA). Advances in this technology have provided other alkanolamine solvents for CO2 treating in various industries. Monoethanolamine (MEA), diethanolamine (DEA), and the hindered amine AMP are used alone in an aqueous solution. Typical solvent blends include a methyldiethanolamine (MDEA) solution promoted by piperazine or other secondary amines. Also, potassium carbonate solvents are commonly promoted by DEA or other reactive amines.
Gas absorption is a process in which soluble components of a gas mixture are dissolved in a liquid. Stripping is essentially the inverse of absorption, as it involves the transfer of volatile components from a liquid mixture into a gas. In a typical CO2 removal process, absorption is used to remove CO2 from a combustion gas, and stripping is subsequently used to regenerate the solvent and capture the CO2 contained in the solvent. Once CO2 is removed from combustion gases and other gases, it can be captured and compressed for use in a number of applications, including sequestration, production of methanol, and tertiary oil recovery.
The conventional method of using absorption/stripping processes to remove CO2 from gaseous streams is described in U.S. Pat. No. 4,384,875, which is incorporated herein by reference. In the absorption stage, the gas to be treated, containing the CO2 to be removed, is placed in contact, in an absorption column, with the chosen absorbent under conditions of pressure and temperature such that the absorbent solution removes virtually all the CO2. The purified gas emerges at the top of the absorption column and, if necessary, it is then directed towards a scrubber employing sodium hydroxide, in which the last traces of CO2 are removed. At the bottom of the absorption column, the absorbent solution containing CO2 (also called “rich solvent”) is drawn off and subjected to a stripping process to free it of the CO2 and regenerate its absorbent properties.
To effect the regeneration of the absorbent solution, the rich solvent drawn off from the bottom of the absorption column is introduced into the upper half of a stripping column, and the rich solvent is maintained at its boiling point under pressure in this column. The heat necessary for maintaining the boiling point is furnished by reboiling the absorbent solution contained in the stripping column. The reboiling process is effectuated by indirect heat exchange between part of the solution to be regenerated located in the lower half of the stripping column and a hot fluid at appropriate temperature, generally saturated water vapor. In the course of regeneration, the CO2 contained in the rich solvent is released and stripped by the vapors of the absorbent solution. Vapor containing the stripped CO2 emerges at the top of the stripping column and is passed through a condenser system which returns to the stripping column the liquid phase resulting from the condensation of the vapors of the absorbent solution. At the bottom of the stripping column, the hot regenerated absorbent solution (also called “lean solvent”) is drawn off and recycled to the absorption column after having used part of the heat content of the solution to heat, by indirect heat exchange, the rich solvent to be regenerated, before its introduction into the stripping column.
In simple absorption/stripping as it is typically practiced in the field, aqueous rich solvent is regenerated at 100-160° C. in a simple, countercurrent, reboiled stripper operated at a single pressure, which is usually 1-2 atm. The rich solvent feed is preheated by cross-exchange with hot lean solvent to within 5-30° C. of the stripper bottoms. The overhead vapor is cooled to condense water, which is returned as reflux to the countercurrent stripper. When used for CO2 sequestration and other applications, the product CO2 is compressed to 100-150 atm.
Commercially used amines that are used by themselves in water as absorbers include monoethanolamine, diethanolamine, methyldiethanolamine, diglycolamine, diisopropanolamine, some hindered amines, and others (Kohl and Nielsen (1997)). These amines are soluble or miscible with water at ambient temperature at high concentrations that are used in the process to maximize capacity and reduce sensible heat requirements. Other amines, including piperazine, are used in combination with methyldiethanolamine and other primary amines.
A number of mono- and polyamines, including piperazine, are identified as potentially useful solvent components but have not been used because they are insufficiently soluble in water when used by themselves. Piperazine is a diamine that has previously been studied as a promoter for amine systems to improve kinetics. In water at 25° C., solid piperazine has a solubility less than 2 M, so it cannot be used in traditional systems at concentrations that give adequate CO2 capacity for good energy performance. BASF has disclosed the use of piperazine in combination with other amines (such as alkanolamines) or highly water soluble organics (such as triethyleneglycol) to promote the water solubility of piperazine as described in Canadian Patent No. 2651265.